Multiple ratio mechanical transmissions, whether manual, automatic, or semi-automatic, utilize a fluid disposed within the transmission to accomplish various functions. For example, manual mechanical transmissions use a fluid for lubrication of rotating and meshing parts. Automatic transmissions use a fluid for generating and transferring hydrodynamic power in addition to providing lubrication for rotating parts. The specific formulation and properties of a transmission fluid are generally adapted to the particular application. However, regardless of the application, the various types of transmission fluids have some common properties.
Since vehicles, such as tractor semi-trailer trucks, are often subjected to extreme temperature variations, it is desirable for a transmission fluid to function somewhat consistently over the vehicle operating range. In addition to variations in ambient operating temperatures, a transmission fluid is also subjected to heat generated by friction among the numerous rotating components. This temperature fluctuation is manifested as a change in viscosity of the transmission fluid. In general, the fluids are more viscous (thicker) at lower temperatures and less viscous (thinner) at higher temperatures. Even multi-viscosity fluids, which are formulated to have a smaller change in viscosity over a substantial portion of their operating range, experience a noticeable change in viscosity when subjected to temperature extremes. The change in viscosity may pose a number of challenges in designing a mechanical transmission for consistent performance over a typical operating range.
Once a transmission fluid is added to a transmission, it is desirable to retain it within the transmission. This is accomplished utilizing a variety of seals, including stationary seals which are interposed between two (2) non-rotating transmission components, such as a transmission case and bottom pan. Rotary seals are interposed between rotating and stationary components, such as an input shaft or output shaft seal. The seals function to retain the transmission fluid within the transmission while isolating the interior of the transmission from exterior contaminants which may cause accelerated wear of transmission components if present. The seals must maintain the integrity of the transmission when exposed to temperature fluctuations and the resulting variation in transmission fluid viscosity.
Ambient and operating temperature variations also result in expansion and contraction of transmission components. Of course, the transmission fluid volume also varies as a result of the expansion and contraction of the transmission fluid. This results in a pressure differential between the interior of the transmission and the exterior which is subjected to atmospheric pressure. Thus, if the pressure within the transmission is greater than the pressure outside the transmission, the various seals are subjected to a force which tends to expel fluid from the transmission. This may lead to a seal failure if the pressure change is rapid enough. Similarly, if the pressure within the transmission is lower than the atmospheric pressure, the seals are stressed in the opposite direction and dirt, water or other contaminants may be drawn into the transmission. Thus, it is desirable to control the pressure differential between the interior and exterior of a transmission.
A pressure differential may also be created by changes in atmospheric pressure as a vehicle travels from a lower elevation to a higher elevation. Although this change may seem insignificant, atmospheric pressure can vary significantly from elevations ranging from below sea level to those exceeding 10,000 feet.
Due to weight and packaging considerations, most mechanical transmissions are designed with a predetermined limited fluid volume. A number of operating problems may develop if the transmission is overfilled with an amount of fluid exceeding design limits. An overfill condition may lead to excessive internal pressure which may cause premature seal failure. When a transmission is overfilled, the fluid which is normally collected in a pan or reservoir may constantly contact the rotating components of the transmission during operation. This may cause excessive aeration resulting in vaporization and foaming of the fluid. The associated pressure increase and degradation of fluid properties may be detrimental to the performance and durability of the transmission.
Thus, a number of transmission manufacturers have added a vent or breather to their transmissions to equalize interior and exterior pressures. For many applications, a small vent tube which is strategically positioned provides this function. However, many of the prior art devices allow bridging of the transmission fluid which results in unnecessary fluid expulsion. Bridging occurs when the transmission fluid spans the opening of the vent or breather due to the cohesion tension of a viscous fluid. This phenomenon is especially prevalent in more viscous fluids, i.e. when the fluids are cold. As the transmission warms up, the pressure increases within the transmission creating a bubble within the tube or breather which forces the fluid to be expelled unnecessarily from the transmission. Thus, it is desirable to provide pressure equalization while reducing or eliminating unnecessary transmission fluid expulsion from the breather during normal operation of the transmission. It is also desirable to reduce or eliminate fluid escape in the form of vapor.